There is a NHLA standard for log grading that assesses log volume or "scale" (accounting for wood that won't turn into lumber) and quality or "grade". If you are not familiar with those rules, and you plan to buy logs in the future you might want to check out Penn State's 2 day course that teaches the rules.
Without looking at the logs, I'd say make an offer and see what happens. When I buy logs, I usually have an idea what I can get for the lumber that it might make, and also what it will cost me to handle, cut, store and dry the lumber, subtracting that off from what I can sell, and accounting for profit and my time, that gives me a number I'm willing to pay for the logs. That number is not always positive, BTW. Meaning it would cost me more to buy and cut the logs than I could possibly make.
Based on what you describe, and the seemingly high demand for Walnut (in my area anyway - Southwest PA), I'd pay somewhere between 20 and 40 cents per BF for walnut of this age, and size, erring towards the lower range for the smaller stuff and those that are deeply checked or cracked.